Vacuum dilution extraction gas sampling system

ABSTRACT

A gas sampling system utilizing a pair of parallel sonic orifices. One of the orifices is connected to a source of filtered, heated samples. The second, larger orifice is connected to a source of dilution gas. A vacuum pump maintains a substantial vacuum behind the orifices, thus assuring critical flow therethrough. The sample and diffusion gas are mixed behind the orifices and transported under partial vacuum for analysis. The dew point of the sample is affected by both the ratio of the diameter of orifices and the degree of vacuum transporting the gas mixture. As such, the dew point can be varied indefinitely by any reasonable combination of orifice ratio and vacuum pump strength.

RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 07/789,935, filed Nov. 12, 1991 now U.S. Pat. No. 5,297,432.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to extractive gas sampling systems suchas are used for analysis of process gases or fossil fuel combustivegases being vented through a stack.

2. Description of the Prior Art

An important category of extractive gas sampling relates to thecompliance monitoring requirements enforced by the United StatesEnvironmental Protection Agency (EPA). Many sources of air pollution,such as fossil-fueled power plants, incinerators, metal smelters, andcement kilns, are required to monitor levels of certain gaseous speciesthat are released into the atmosphere. These species include sulfurdioxide, nitrogen oxides, carbon monoxide, carbon dioxide and oxygen.The EPA standards for compliance monitoring systems are delineated inVolume 40 of the Code of Federal Regulations.

The gas streams to be monitored typically have certain intrinsiccharacteristics which complicate testing. For example, they generallycontain 6% to 20% by volume of evaporated moisture, which results in asample dew point well above that of normal ambient temperatures. Also,the gas streams often contain significant amounts of condensed moisturein the form of entrained water droplets and fog. Acid gases, such assulfur dioxide are also generally present. Additionally, the gas streamsinvariably contain large quantities of particulate debris such as soot,fly-ash from fossil fuels and process material.

In order to analyze a sample for its gaseous constituents, it isnecessary to remove the particulates and transport the sample to aremote location suitable for the operation of gas analysisinstrumentation. For accurate measurements and for reliability of thetest equipment, it is necessary to ensure that moisture and gases willnot condense either in the sample probe, the sample lines, or theanalyzers. However, the methods used to accomplish these goals must notthemselves alter the samples in a way that negatively impacts nestingaccuracy. In the past, two basic types of sampling systems have beendeveloped for analysis of gaseous mixtures.

The first type, the traditional extractive system, is shown in FIG. 1.Many vendors have supplied similar systems over the years. This system,however, has proved to have many undesirable drawbacks as describedbelow.

The second type, illustrated in FIG. 2, is a venturi dilution probesystem. This type of system was developed in the 1980s primarily inresponse to perceived inadequacies with the traditional system. Asdiscussed more fully below, however, the venturi probe system is alsonot without disadvantages.

SUMMARY OF THE INVENTION

A gas sampling system practicing the present invention utilizes a pairof sonic orifices. One of the orifices is connected to a source offiltered, heated samples. The second, larger orifice is connected to asource of dilution gas. A vacuum pump maintains a substantial vacuumbehind the orifices, thus assuring critical flow therethrough. Thesample and diffusion gas are mixed behind the orifices and transportedunder partial vacuum for analysis. The dew point of the sample isaffected by both the ratio of the diameter of orifices and the degree ofvacuum transporting the gas mixture. As such, the dew point can bevaried practically indefinitely through an optimum combination oforifice ratio and vacuum pump strength.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a prior art system utilizing a heated sample linefor transporting the sample.

FIG. 2 is a diagram of a prior art system utilizing a dilution probe.

FIG. 3 is a diagram of a present preferred embodiment of the vacuumdilution extractive gas sampling system of the invention.

FIG. 4 is a perspective view, partially cut away of a present preferredprobe for use with the invention.

FIG. 5 is a cross-sectional view taken on a plane extending from line5--5 to the end of the probe shown in FIG. 4.

FIG. 6 is a side view partially in section of a second preferredembodiment of the vacuum dilution extractive gas sampling system.

FIG. 7 is a cross-sectional view of the probe end of the embodiment ofFIG. 6.

FIG. 8 is an end of the embodiment of FIG. 6 with the cover open.

FIG. 9 is a plan view partially in section of the mixing chamber.

DETAILED DESCRIPTION Prior Art Systems

FIG. 1 illustrates the traditional extractive system. Pump 10 draws gasthrough heated probe 12 from a gas stream moving within stack 14 asshown by arrow A. The sample is then transported to a remote locationthrough a heat-traced sample line 16. Typically, probe 12 and sampleline 16 are heated to about 250° F. to prevent condensation of themoisture or acid in the sample. Next, the sample is drawn through a"chiller" 18 which lowers the sample temperature to approximately 35° F.The water vapor thus condenses and is drained away at drain 20. Thesample, now dry, is then reheated and transported through analyzer 22which measures the constituents of interest. The gas sample ismaintained at or near atmospheric pressure during all of this process.

A number of minor variations have been made on this basic design.Sometimes the pump is located before the analyzer. Sometimes the gassample is diluted via the addition of nitrogen or air prior to analysisfor the purpose of bringing the sample concentration within the range ofthe analyzers or for the purpose of reducing certain interferenceswithin the analyzers. The analyzer and sample pump are sometimes heatedso that the chiller may be omitted.

This traditional design presents a number of drawbacks and limitations.First, in order to move the sample to a remote location within anacceptable time period (EPA requires a 15 minute system response time,and process applications demand even a quicker response) the sample mustbe large--typically in the order of two to five liters per minute.Because the amount of particulate associated with such large sampleswould quickly clog any fine filter, only a coarse filter 24, such as thetype constructed of sintered metal or ceramic, can be used. Even acoarse filter, however, will tend to clog every few hours in thissystem. To clean the filter, a blow-back design is required. For thispurpose, compressed air source 26 feeds surge tank 28 which is locatednear ball valve 30. When valve 30 opens pressurized air in tank 28 isreleased, thereby purging filter 24 of impurities. Since valve 30 iscontinually exposed to the stack sample, however, it can develop leakswhich distort the sample.

This design also requires use of large amounts of calibration ("cal")gas. Cal gas is a gas sample containing a known concentration of thespecies to be measured. This is used to run a calibration check on theaccuracy of the measuring equipment. The EPA requires that such acalibration check be run daily using "Protocol-1" gases that maytypically cost $400 for a small bottle. A similar technique using "zerogas" is sometimes employed to null the species detectors. Referringagain to FIG. 1, the cal gas is fed in the traditional design from calgas source 32 through line 34 to a location on probe 12 which is behindfilter 24. Thus, deleterious effects of filter 24 such as scrubbing ofsulfur dioxide by alkali particles thereon are not checked by the calgas. In addition, a large volumetric flow of cal gas greater than thevolumetric flow through tube 16 is required.

The design has a number of "weak links", which make it inherentlyunreliable. For example, if chiller 18 fails, analyzer 22 and pump 10will likely be destroyed. Additionally, failure of heat tracing sampleline 16 will result in condensation and contamination that cannecessitate replacement of the line and all downstream plumbing. Heattraced line is significantly more expensive than unheated line. Also,since ball valve 30, analyzer 22 and pump 10 are exposed to high levelsof acidic gases and to the fine particulates which permeate coarsefilter 24, the service life of these components is reduced considerably.

Furthermore, when this design utilizes a chiller, a serious measurementmethodology problem is presented. Specifically, gas concentrations aremeasured on a dry basis (i.e. with the moisture removed). Pending EPAregulations strongly favor making the concentration measurement on a wetbasis (including vapor-phase moisture).

The second general type of prior art system, the dilution probe, isdepicted as FIG. 2. In this design, the rate of stack sample extractionis considerably smaller than is the case with the traditional system ofFIG. 1. Here, gas is drawn through a fine filter 35 into a device knownas a "sonic orifice" or "critical-flow orifice." Sonic orifice 36 is socalled because it meters a constant volumetric flow provided that asubstantial vacuum exists behind the orifice. Stated another way, apressure drop of greater than two to one (2:1) thereacross will induce agenerally constant volumetric flow as metered from the upstream side ofthe orifice. Orifice 36 can typically be sized to permit flow as low as20 cc per minute and as much as 200 cc per minute. Vacuum on the backside of orifice 36 is maintained by a venturi 38 which is driven bycompressed air source 40. Venturi 38 also serves to provide clean, drydilution air which lowers the sample point. The entire venturi/orificeassembly is constructed within nonheated probe 42 such that the dilutionis accomplished at essentially stack temperature. The diluted sample isthen sent to analyzer 43 at approximately atmospheric pressure.

This technique overcomes some of the deficiencies of the traditionalextraction system. For example, cal gas 44 and zero gas 45 may beintroduced upstream of filter 35 which will allow checking ofdeleterious filter effects. However, significant drawbacks remain. Forexample, because orifice 36 is a true critical-flow device, whileventuri 38 is not, the dilution ratio is a function of temperature. Ifthe process temperature varies considerably, the probe will need to betemperature controlled. Additionally, if the gas stream being sampled isfully saturated, condensation will occur on filter 35 and orifice 36before dilution can occur. In addition, condensation will occur justdownstream of the orifice 36 due to adaibetic cooling of gas passingthrough. In these applications, it therefore is necessary to heat theprobe anyway.

Furthermore, in order to prevent condensation in unheated transport line46, it is necessary to lower the dew point to below the expected ambienttemperature. In cold climates, dilution ratios of up to 350:1 areneeded. Ratios of this magnitude pose several problems. First, theconcentration of the gas constituents of interest may be lowered to alevel below the sensitivity of commercially available analyzers. Forexample, the best carbon monoxide analyzers can only measure down tofive parts-per-million (5 ppm) with good accuracy. Many facilities mustmeasure actual stack concentrations of the order of 50 ppm. Stack gashaving about 50 ppm of a constituent diluted by the dilution ratioachieved in the prior art system of FIG. 2 reduces the concentration towell below 5 ppm. Another problem with high dilution ratios is that theoverall system will become sensitive to minute impurities in thedilution air. As an illustration, 0.1 ppm of CO in the dilution air ofthe above example will be measured by the system as (350)×(0.1 ppm), theproduct of which is thirty-five parts per million (35 ppm). The analyzerwill be unable to differentiate between this error and a comparablestack level of CO.

Moreover, the only commercially available version of this device uses aventuri that is operational only with flows of between four and sevenliters per minute. This also poses several problems. For example, thislarge a flow of the dilution gas effectively militates against the useof bottled gas which would be prohibitively expensive and requirefrequent maintenance. Thus, compressed air source 40 is a compressorwhich utilizes the air in or near the stack. That air containsparticulates, CO₂, SO_(x), NO_(x) and water vapor. Consequently, onemust use an array of dryers 401, scrubbers 402, absorbants 403 andfilters 404 to remove contaminants from the dilution gas. Since mostanalyzers only require a flow in the order of 0.5 liters/minute, most ofthe 4-7 liters of diluted sample are wasted. Another problem is that,for a given sized orifice, there are limits to the dilution ratios thatcan be achieved.

Additionally, venturi 38 is generally embedded in a very expensive probeassembly. Thus, contamination, such as could occur if the orificeassembly, which is typically made of glass, would break, necessitatesreplacement of a very expensive piece.

Gas sampling systems have also been used to evaluate exhaust emissionsfrom internal combustion engines. Examples of such systems are disclosedin U.S. Pat. Nos. 3,817,100 to Anderson et al.; 3,965,749 to Hadden etal.; 4,823,591 to Lewis, and 5,184,501 to Lewis et al. None of thesedevices are suitable for delivering a diluted sample over longdistances. They also use valve and orifice combinations which do notprovide constant dilution ratios.

DESCRIPTION OF PRESENT PREFERRED EMBODIMENTS

In accordance with the present invention, a gas sampling extractionsystem is provided which overcome many of the disadvantages of theexisting technology. The system utilizes a pair of sonic orifices. Oneof the orifices provides a constant flow of sample gas and the otherprovides a constant flow of dilution gas. The resulting mixture istransported under substantial vacuum and repressurized to typicallyabout one atmosphere prior to analysis.

FIG. 3 illustrates a present preferred embodiment of the gas samplingextraction system of the invention. Gas from a system such as a gasstream moving within stack 47 is drawn by sample pump 48 into collectionprobe 50. The gas is preferably first filtered. Next, the sample passesthrough capillary tube 56 which is within probe 50. After leaving tube56, the sample gas enters sample conduit 57 which has sample orifice 58therein. Dilution gas is preferably simultaneously drawn at a controlledpressure by pump 48 from dilution gas source 60 into dilution gasconduit 62 and through dilution orifice 64 therein. Typical suitabledilution gases may be compressed air, carbon dioxide and nitrogen,depending on the sample gas and the analyzers which are desirable to beused. In order for the sample and dilution gas to be drawnsimultaneously, orifices 58 and 64 are arranged in parallel.Specifically, conduits 57 and 62 intersect downstream of the orifices,forming mixing conduit 67 where mixing of the sample and dilution gasoccurs. Since, pump 48 maintains a substantial vacuum in conduit 67, theflow rate through orifices 57 and 62 is essentially constant. Thus, aconstant dilution ratio is achieved.

Conduits 57, 62 and 67 may be constructed of any suitable inertmaterial. Some possible materials for this purpose are glass or acorrosion resistant metal alloy such as Hastalloy® corrosion resistantalloys or corrosion resistant polymeric materials such as Teflon®material. Particularly, Hastalloy® C-22 may be suitable. Orifice 58 maybe as small as 0.0009 inches which corresponds to a flow rate of 4.2 ccper minute. This is much less than the minimum 20 cc per minute used inthe venturi dilution system shown in FIG. 2. For a dilution ratio of25:1, dilution orifice 64 must be five times larger in diameter thanorifice 58, or 0.0045 inches in this example. This gives a flow rate of105 cc per minute of dilution gas if the dilution gas is delivered atbeneficial pressure. If the dilution gas is provided at a higherpressure, e.g., 14.7 psig, the flow will be 211 cc per minute and thedilution ratio will be 50:1. As this is a much smaller rate than theprior art, it is possible to use bottled dilution gas from gas cylindersinstead of plant instrument air or compressor air. This completelyeliminates problems with contamination in the dilution gas. Furthermore,by suitable selection of the orifices, it is possible to achieve anydesired volumetric dilution ratio over a range of 1:1 to 250:1. For anyspecific set of orifices, it is possible to adjust the volumetricdilution ratio by a factor of 10, that is, over a 1:1 to 10:1 range, bysimply adjusting the dilution gas pressure. Thus, the system enjoys alevel of flexibility previously unattainable.

It is desirable to maintain orifices 58 and 64 in a temperaturestabilized dilution chamber such as heated chamber 66. Chamber 66 ismounted engaging mounting nipple 68 which protrudes from wall 70 ofstack 47. If the temperature in the dilution chamber is maintained at afairly constant figure, the dilution ratio will be impervious to stacktemperature variation. A temperature of 250° F. has been found suitablefor this purpose since it is well above the dew point of most stackgases.

Typically, the dilution ratio should be chosen such that the dew pointis lowered to below 30° F. Dew points below this temperature aregenerally not harmful to the analyzing equipment since such equipmentoperates at a higher temperature. Thus, condensation within the analyzerrange will not occur. Generally, dilution ratios between 10:1 and 50:1will accomplish this dew point lowering. However, a dilution ratio ofeven 50:1 will generally not alone lower the sample dew point enough foruse in a cold climate since conduit 67 may frequently be exposed totemperatures below 30° F. For this reason, sample pump 48 transports themixture under a substantial vacuum (0.15 atmospheric pressure) whichfurther lowers the dew point of vapor by the factor (1/0.15)=6.67. Thecombination of actual volumetric dilution in vacuum transport makes itpossible to lower the dew point below the coldest expected ambienttemperature without actually reducing the relative concentration of thespecies of interest to the undesirably low levels of the prior artdevices. Specifically, it is generally possible to easily lower the dewpoint to -25° F. using this technique.

The sample mixture next enters the vacuum side of sample pump 48 andexits the pressure side of sample pump 48 at essentially atmosphericpressure. It is not essential, however, that the analyzer 69 be placedon the pressure side of pump 48 and in other applications it may bedesirable to place the analyzing equipment on the vacuum side of thepump. Thus, the species of interest are presented to analyzer 69 at adilution ratio of only approximately 50:1 in the present example. To getthe same dew point lowering with the prior art devices, the sample wouldhave been presented to the analyzer with a dilution ratio ofapproximately 50:1×6.67=333. This would be an unacceptably high dilutionratio in many cases for the reasons discussed above. Another advantageof transporting the sample under substantial vacuum is that rapidmovement of the sample may be accomplished with a smaller sample rate.That is to say at reduced pressure, a sample of approximately 211 cc perminute will move through the sample line as fast as a much larger sample(211×6.67) cc/min would have moved at barometric pressure.

The preferred transport pressure for the system shown in FIG. 3 is 0.15atmosphere or lower. Commercially available sample pumps could operateat as low as 0.075 atm. The use of more expensive sample pumps capableof achieving these lower pressures, however, is only necessary in theevent that: (1) the actual dilution ratio must be kept low out of a needto operate on the range of a specific analyzer; and (2) a low ambienttemperature is expected. In a warm climate it will be possible toachieve a sufficiently low dew point in conduit 67 with both a lowdilution ratio and a relatively inexpensive pump. For purposes ofachieving critical flow the pump only needs to achieve a vacuum ofapproximately 0.4 atm pressure.

FIGS. 4 and 5 illustrate a presently preferred embodiment of acollection probe 50 practicing the present invention. Probe 50 comprisesa generally hollow cylindrical body 72 which may typically be from threeto six feet in length. An end cap 74 is mounted on the end of body 72which will be exposed to the gas stream. Inside of body 72 are samplegas capillary tube 56 and cal gas capillary tube 76. Typically, thesetubes would be constructed of glass, Teflon polymer or other inertmaterial. Tube 56 and 76 both extend into filter holder which containsfine filter 54. Tube 56 terminates just downstream of filter 54 whiletube 76 extends through filter holder 55 to a point just upstream offilter 54. If tubes 56 and 76 are made of glass or other fragilematerial, they are maintained in position within body 72 by a series ofspacers, such as spacer 78. These spacers are typically constructed of aresilient material, such as Teflon® material, in order to dampvibrations and the like which could damage the tubes.

A mounting flange 80 is attached to the other end of body 72. Mountingflange 80 is adapted to engage a mounting nipple, such as nipple 68 ofFIG. 3, to maintain probe 50 in position. Also at this end of probe 50are the various plumbing connections between sample probe 50 and therest of the system. Sample conduit 57 is shown coupled to dilution gasconduit 60 using a T-coupling 82. The mixture then exits through mixingconduit 67 to pump 48 and eventually to at least one analyzer 69. Afterleaving analyzer 69, the sample is generally vented to the atmosphere.Zero gas and cal gas enter tube 76 through zero gas conduit 84 and calgas conduit 86. As discussed above, all of these connections are housedwithin a temperature controlled chamber 66 (FIG. 3) to maintainapproximately constant flow rates. An inner wall 88 of chamber 66 isshown in FIG. 4. Wall 88 is attached to mounting flange 80 using boltssuch as bolt 90.

The interior construction of the distal end of the probe is illustratedin FIG. 5. Cap 74, which is typically made of Teflon® material, containsa number of leakage paths 96 through which, for example, furnace fluegas in a stack is introduced to sample channels 97. Channels 97 aresituated within filter holder 55 to the front of fine filter 54. Cap 74and its associated leakage paths 96 effectively constitute an inertialfilter for separation of the gas to be measured from entrained waterdroplets and large particulates. Leakage paths 96 and sample channels 97additionally constitutes of a coarse filter for additional removal ofentrained water droplets and large particulates. Gas then passes throughfine filter 54. Fine filter 54 may be constructed of quartz wool or thelike. Next, the sample passes through channel 98 to capillary tube 56.Although not evident in FIG. 4, support tube 93, which is typically madeof Teflon® material, surrounds tube 56 to provide mechanical support atthis point. Similarly, support tube 94 supports cal gas capillary tube76. Support tubes 93 and 94 are maintained in position by spacer 78.Though not shown, support tubes 93 and 94 extend the full length of body72.

Heating means, such as heater wire 92, which is typically made of 316stainless steel or suitable corrosion-resistant material, may beprovided in the case in which the stack gases are fully saturated withwater vapor or other condensible material. Wire 92 serves to heat thecavity of filter holder 55 in which fine filter 54 is located. Heaterwire 92 further serves to warm sample gas capillary tube 56 and cal gascapillary tube 76 to prevent condensation in either region. Wire 92 thenforms a coil 99 around the outside of filter holder 55 by enteringthrough the annular space between tube 56 and support tube 93 andreturns similarly through the space between cal gas tube 76 and itssupport tube 94. Filter holder 55 may be made of Teflon® material and isfurther equipped with O-rings, such as 95, which protect wire 92 and thegeneral interior region of probe 72 from corrosive attack by acid gasesand from condensation of stack moisture.

It can thus be seen that a novel system of extracting gas sampling forthe analysis of process gases or fossil fuel combustive gases has beenprovided. The dew-point lowering required for sample transport withoutheated lines is accomplished by a combination of volumetric dilution andtransport under substantial vacuum. Other benefits include reduced usageof dilution gas sufficient to permit use of bottled dilution gases,measurement of species that could not be measured were the volumetricdilution too high, fast response time, and flexibility in settingdilution ratios.

Referring now to the second embodiment shown in FIGS. 6 thru 9, weprovide a housing 100 with cover 101 which contains a mixing conduit ordilution chamber 102. A probe housing 104 extends from the main housing100. The device is fitted into an opening in a stack indicated by stackwalls 103 shown in chain line. At the distal end of the probe assemblywe provide a removable end housing 106. That housing 106 consists of amain body portion 107 from which an annular flange 108 extends. A samplegas channel 109 and calibration gas channel 116 are provided through themain body 107. The stack gas sample is collected and flows throughchannel 109 and conduit 114 to the mixing chamber. Sample conduit 114 isinserted into counterbore 170 in the end housing 106 Similarly,calibration gas conduit 116 is inserted into counterbore 171. A pair offerrule-type bulk head valves 111 and quick release couplings 112connect the end housing to sample conduit 114 and calibration gasconduit 116. Insulating sleeeves 117 are provided around the full lengthof sample conduit 114 and calibration gas 116. A heater wire 118 passesthrough the insulating sleeve 117 and surrounds the body 107 of theremovable end housing 106. A heater conduit 119 permits the heater wire118 to pass through the end housing 106 and then be coiled as coilsection 118. The heat wire 118 maintains the sample at a hightemperature, preferably about 250° F., until the sample reaches thedilution chamber 102 which is also heated. A course mesh filter 120 ispositioned over a collection orifice 121. Preferably this filter issized at from 0.05 to 0.15 mesh. Within collection orifice 121 weprovide a glass wool filter 122 and a Teflon filter 124. The mesh filter120 is preferably made of Teflon polymer or other hydrophobic material.Its function is to collect and coalese moisture in the sample passingthrough the filter which moisture would then drip from the mesh filter.Glass wool filter 122 and Teflon filter 124 prevent particulates fromentering into the sample channel 109. The annular collar 108 surroundingthe end of the probe keeps rain and other material which may be fallingdown the stack away from the collection orifice and filters 120 and 122.We prefer to provide O-ring seals 125 between the end housing 106 andprobe housing 104.

The sample conduit 114 and cal gas conduit 116 are telescopicallyconnected to the main housing at connections 126. Thus, it is arelatively simple matter to pull the end housing 106 from the probehousing 104 and disconnect the couplings 112 to remove end housing 106from the system. Consequently, it is relatively easy to routinely cleanor replace the filters used in this portion of the probe as well as thesample conduit 114 and cal gas conduit 116.

Within housing 100 the sample conduit 114 continues to form a coil 128which then is coupled to dilution chamber 102 within a housing 146. Calgas conduit 116 continues to form coil 129. Coil 129 is within coil 128and line 116 established line 114 in housing 110. Therefore, coil 129and line 116 can only be seen in the housing from the view of FIG. 8. Acartridge heater 132, shown in chain line in FIG. 8, is provided in thehousing 146 to maintain the dilution chamber 102 at an elevatedtemperature. Also, provided within the housing is a temperature probe orthermister 134 which monitors the temperature of the dilution chamber102. Thus, the sample is maintained at an elevated temperature from thepoint of collection through dilution in the dilution chamber. A buss 136is provided within the housing 100 to make electrical connections tocartridge heater 132 and thermister 134 as well as to the heater wire118 which surrounds the sample conduit 114. Power is supplied to thebuss 136 by cable 138 which also carries wires from the temperaturegauge and heater controller which are normally located near the analyzerremote from the collection probe.

In FIG. 9 there is shown the interior of the dilution chamber 102. A gassample enters the dilution chamber through conduit 114. As the gasapproaches the conduit it first goes through a filter 150, passageway151 and first critical flow orifice 152a in orifice plate 152. Then thesample passes through a second coarse filter 154. Calibration gas entershousing 146 through line 142. The diluted sample is removed from thedilution chamber through line 141. An O-ring seal 153 is placed betweenfilters 152 and 154. These filters and orifice plates are preferablymade of a corrosion resistant alloy, such as Hastelloy 276 alloy. Fromthe opposite end of the dilution chamber dilution gas enters throughconduit 140. The dilution gas passes through a filter and heat exchanger160, passageway 161, second critical flow orifice 162a in orifice plate162, coarse filter 164. An O-ring 163 is placed between the filter 164and orifice plate 162. Filters 164 is preferably made of the samecorrosion resistant alloy as filters 150 and 154. The larger filter 160is made of a heat conductive material such as a copper alloy andfunctions to heat the dilution air as it enters the dilution chamber.The first and second critical flow orifices are sized to maintain aconstant dilution ratio independent of minor fluctuations in the vacuumbeing drawn through conduit 141. We prefer to size the first criticalflow orifice 152a at 0.0012 inches in diameter and the second criticalflow orifice 162a at 0.006 inches in diameter.

It will be recalled that the entire housing 146 containing mixingchamber 102 has been heated by a cartridge heater 132 shown in chainline in FIG. 8. Within the dilution chamber 102 the dilution gas andsample gas mix to a desired ratio. Then the mixture is drawn throughconduit 141 by a vacuum pump (not shown) connected to the sample line141. We prefer that the dilution chamber be maintained at subatmosphericpressures preferably ranging from 0.5 to 0.15 atmospheres to ensurecritical flow through the orifices 152a and 162a and also to ensurerapid transit of the sample from the mixing chamber to the anlayzer andto maintain a low dew point. As previously mentioned, within thedilution chamber we usually achieve dilution ratios ranging from 10:1 to100:1 dilution gas to sample gas. The dilution chamber as well as thesample lines are made of non-corrosive materials which are suitable forwithstanding the elevated temperatures encountered during operation.Preferably, these materials also will not be adversely affected bytemperatures normally encountered in the environment should the probenot be in use and the heating elements not have been activated.

As certain presently preferred embodiments have been described and shownherein, it is to be understood that various other embodiments andmodifications can be made within the scope of the following claims.

We claim:
 1. An apparatus for collecting a sample stream from a systemcontaining at least one gas comprising:a) a collection probe adapted todraw a portion of at least one gas from the system; b) a first conduitconnected to the probe and having a first critical flow orifice thereinthrough which the portion of at least one gas is drawn from the system;c) means for supplying a dilution gas; d) a second conduit connected tothe means for supplying a dilution gas and having a second critical floworifice through which the dilution gas is drawn; e) a dilution chamberattached to the first and second conduits in which the portion drawnfrom the system and the dilution gas are mixed to form a mixture; f) athird conduit connected to the dilution chamber through which a dilutedsample is drawn; and g) at least one pump connected to the third conduitfor drawing a vacuum on the connect mixture and repressurizing themixture, the vacuum being sufficient to induce critical flow through thefirst critical flow orifice and the second critical flow orifice whereinthe first and second critical flow orifices are sized to maintain aconstant dilution ratio independent of minor fluctuations in the vacuumbeing drawn.
 2. The apparatus of claim 1 also comprising a filterattached to the collection probe.
 3. The apparatus of claim 1 whereinthe dilution gas is at least one compressed gas cylinder.
 4. Theapparatus of claim 1 also comprising means for supplying at least onecalibration gas connected to the collection probe.
 5. The apparatus ofclaim 1 also comprising means for mounting the collection probe in astack.
 6. The apparatus of claim 1 also comprising at least one analyzerconnected to the third conduit.
 7. The apparatus of claim 1 wherein thevacuum creates a pressure in the mixture not greater than 0.40atmospheres.
 8. The apparatus of claim 1 wherein the first critical floworifice is 0.0009 inches and the second critical flow orifice is 0.0045inches.
 9. The apparatus of claim 1 wherein the first and secondcritical flow orifices and the dilution chamber are made of corrosionresistant material selected from the group consisting of corrosionresistant metal alloys, glass, and corrosion resistant polymericmaterials.
 10. The apparatus of claim 1 also comprising at least oneheating means attached to at least one of the first conduit, the secondconduit and the dilution chamber.
 11. The apparatus of claim 1 alsocomprising at least one analyzer connected to the third conduit.